This invention is related to the manufacture of iron casting and, specifically, it refers to a molding sand for casting, suitable for manufacturing cores and chill molds, incorporating hollow microspheres of aluminum silicate.
The iron casting obtained by using cores manufactured with molding sand, generally have a series of defects in their shape, such that it is necessary to subject them to machining to obtain a dimensionally correct piece. These defects are produced due to the heating the core suffers due to the effect of the molten metal poured over it, provoking its expansion and hence, the appearance of fissures on its surface. The molten metal penetrates these fissures, hence forming a kind of partition wall or laminas on the surface of the piece obtained. This undesired effect is known xe2x80x9cveiningxe2x80x9d or xe2x80x9crat""s tailxe2x80x9d.
At present, the cores are manufactured using molding sands and gas- or heat-cured resins, or self-curing resins, together with additives destined to improve the characteristics of the piece obtained.
To prevent the formation of xe2x80x9cveiningxe2x80x9d, a series of techniques are known and used, such as:
The iron oxides used as additives, are destined to minimize the problems created by the expansion of the silica contained in the sands, being used for such a purpose red, black, yellow iron oxides or iron oxide from Sierra Leone, which are incorporated to the mixture in percentages varying from 1 to 3%. These oxides act as a factor for the formation of feyalite, such that the xe2x80x9cveiningxe2x80x9d is minimized during the formation of the fissure. Nevertheless, this technique besides not eliminating xe2x80x9cveiningxe2x80x9d in some cases, has as a disadvantage that the iron oxide reduces the mechanical resistance of the core and moreover the formation of feyalite increases the tendency to penetration, causing the external surface of the piece obtained to present irregularities, which should be treated later.
According to this technique, wood flour or coal powders are added in proportions varying from 1 to 3%. These flours burn during melting, hence leaving free gaps distributed throughout the volume of the core, permitting that the expansion of the silica is produced in these gaps without the need to increase the external size, hence avoiding the appearance of fissures provoking xe2x80x9cveiningxe2x80x9d. The main disadvantage of this technique is that when the flours burn, a large amount of gas is produced which, on circulating, may result in dimensional problems in the pieces obtained. Likewise, with this type of additive, a reduction in the mechanical resistance of the cores is produced.
This new technique described in the U.S. Pat. No. 4,735,973, is based on the use of titanium oxide additives, the additive being present at percentages varying between 0.5 and 5% of the total amount of sand and said additive containing between 15 and 95% titanium oxide. With this technique, thermal expansion is reduced, preventing, as a result xe2x80x9cveiningxe2x80x9d, maintaining the mechanical resistance of the cores and not producing an increase in gas production. The disadvantage of this technique lies in the fact that the cores obtained possesses a certain tendency to penetration, it being necessary to apply paints or other treatments on the surface of the cores obtained before proceeding to melting the piece.
This new technique uses for the formation of the core, special sands of the rounded of sub-angular silica type, chromate sands, zirconium sands and olivine sands, which, due to their different degrees of thermal expansion, result in the reduction of xe2x80x9cveiningxe2x80x9d, and even to its total elimination. The basic disadvantage of this technique is the high cost of this type of sand, with the consequent increase in the cost to obtain the cores.
According to this technique, the silica sand normally used for the manufacture of cores is melted in electric ovens, until obtaining a kind of paste without expansion capacity. Then, the paste obtained is ground until obtaining a sand powder which is mixed approximately at 50% with silica sand. In this way, the expansion of the core is avoided, since the powder obtained from the silica paste does not have a capacity for expansion and hence, neither produces fissures nor the corresponding veining. The basic disadvantage of this technique is the greater complexity of the production process, which makes the final cost to obtain the cores more expensive.
As may be appreciated, the techniques normally used to prevent the formation of xe2x80x9cveiningxe2x80x9d consist either in the use of additives (iron oxide, titanium oxide, wood flours and coal powder) or in the use of special sands (natural sands of low expansion or electrofused sands of low expansion).
Now it has been found that it is possible to improve the quality of the iron casting by using cores or molds manufactured with molding sands incorporating hollow microspheres of aluminum silicate.
As a result, a purpose of this invention comprises a molding sand for casting which incorporates hollow microspheres of aluminum silicate.
An additional purpose of this invention comprises a process to manufacture cores or chill molds including the use of the molding sand indicated above. The resulting cores and molds also comprise a purpose of this invention.
Another additional purpose of this invention comprises a process to manufacture iron casting including the use of the cores or molds mentioned above. The resulting iron casting also comprises a purpose of this invention.
The invention provides a molding sand for casting which incorporates hollow microspheres of aluminum silicate in an amount between 1 and 30% by weight with respect to the total amount of molding sand.
The molding sand, purpose of this invention, is suitable to manufacture cores and chill molds which, in turn, may be used in the manufacture of iron casting.
The use of hollow microspheres of aluminum silicate prevents the appearance of fissures during core expansion, but without increasing gas production and maintaining the mechanical properties of the core obtained. During melting of the piece, the expansion of the silica in the molding sand does not cause an increase of the core, but the expansion is absorbed by the internal spaces of the hollow microspheres, by which the appearance of fissures on the core surface is totally prevented and, as a result, xe2x80x9cveiningxe2x80x9d.
With the molding sand of the invention, cores or molds are obtained of lesser density, by which gas production is reduced, but without decreasing its mechanical resistance. Likewise, the penetration of the piece obtained is reduced, due to the fact that the hollow microspheres of aluminum silicate cover the interstitial spaces of the core producing an effect similar to that of paint, improving the surface of the piece obtained. Therefore, the quality of the resulting iron casting is improved due to the reduction of the defects caused by core expansion and gas production.